In commercial and industrial buildings which utilize significant quantities of incoming external air to ensure that the air within the building is properly cycled, it is conventional to utilize an air-to-air heat exchanger to extract heat from the warm building air which is being discharged so as to effect some preheating of the cold exterior air which is being supplied into the building. Such heat exchanger traditionally employs a boxlike structure having a first pair of opposite sides which define the respective inlet and outlet for the incoming cold air, and having a second pair of sides which respectively define the inlet and outlet for the outgoing warm air. The first and second pairs of sides are generally in transverse or perpendicular relationship to one another. Further, the heat exchanger includes interior dividers, such as a series of parallel plates, which define a first series of channels which extend between the cold air inlet and outlet and permit flow therethrough solely of the incoming cold air, with the dividers or plates also defining a second series of channels which extend between the warm air inlet and outlet and permit flow therethrough of solely the warm air. The first and second series of channels are isolated from one another, but alternately positioned, to permit heat transfer from the warm air to the cold air.
In known air-to-air heat exchangers of the above type, while such heat exchangers function in a desirable manner so long as the outside air temperature does not approach or fall below freezing temperature, nevertheless it has been observed that frost or ice can rapidly build up and either partially or totally seal off the heat exchanger when the outside air temperature is significantly below freezing. Such icing has been observed to occur primarily, and at least initially, at the corner of the heat exchanger defined between the cold air inlet and the warm air outlet. At such corner, the warm air, which may bear a substantial quantity of moisture, can be cooled to below freezing temperature and hence the moisture condenses and freezes up on the heat exchanger plates, thereby closing off some of the channels associated with the warm air. In fact, under severe conditions, it has been observed that the ice will spread across and close off all of the channels associated with and adjacent the warm air outlet.
To avoid or compensate for the above problem, it has been conventional to provide the heat exchanger with some type of arrangement to permit defrosting of the heat exchanger on a controlled or periodic basis. Needless to say, such icing of the heat exchanger presents a formable problem since, even under a partially iced condition, the volume of warm air passing through the heat exchanger decreases so that the capacity and efficiency of the heat exchanger rapidly decreases.
In one known structure which has attempted to compensate for the icing problem, a temperature sensor is located at the corner of the heat exchanger which tends to initially ice up to sense the temperature of the discharged warm air. This temperature sensor is set to sense freezing temperature, namely 32.degree. F. If this temperature is sensed so as to indicate a potential or actual icing problem at this corner of the heat exchanger, then the sensor controls dampers associated with the cold air inlet so that the cold air inlet is effectively closed off, and the cold air is bypassed around the heat exchanger. In this manner, the warm air being supplied to the heat exchanger is effective to deice the heat exchanger and permit the overall temperature thereof to rise, following which the dampers are again adjusted so that the cold air can again be supplied to the cold air inlet. This arrangement, while effective in either deicing or preventing icing of the heat exchanger, is nevertheless undesirable since it results in the heat exchanger efficiency being decreased. Further, this arrangement permits significant volumes of incoming cold air to totally bypass the heat exchanger at irregular intervals and be supplied into the building, and hence this seriously disrupts the uniform temperature of the air within the building.
In another known system which attempts to compensate for this problem, the heat exchanger is controlled by a timer which periodically shuts down the heat exchanger at regular intervals so as to permit defrosting, during which intervals the incoming cold air totally bypasses the heat exchanger. This arrangement, however, possesses the same disadvantages noted above.
In another known system, which has only recently been commercially introduced, an attempt has been made to solve the icing problem by providing a small gate which is mounted at the cold air inlet and which slowly traverses across the inlet so as to periodically close off selected cold air channels for a predetermined period of time. As this gate slowly traverses across the cold inlet, it closes off only a small number of channels at a time, for a selected period of time, whereby the warm air passing through the adjacent warm air channels can defrost the closed-off cold air channels. With this arrangement, however, some of the cold air channels are closed off at all times, and hence the overall capacity and efficiency of the heat exchanger, compared to the total number of flow channels provided, is reduced. Further, this arrangement mounts the gate on a pair of rotatable leads screws, and hence the overall arrangement is mechanically complex and increases the required amount of routine service and maintenance.
Accordingly, it is an object of the present invention to overcome the problem of icing as associated with an air-to-air heat exchanger, and at the same time provide a system which is believed to improve upon and overcome the disadvantages associated with prior known systems as described above.
More specifically, in the improved air-to-air heat exchanger of the present invention, there is provided a flow control arrangement directly adjacent the cold air inlet, which flow control arrangement can be controlled to partially close off, to a varying extent, the cold air inlet in the vicinity of the corner which typically ices up. The control apparatus, which in a preferred embodiment is responsive to outside temperature, can be adjustably positioned relative to the cold air inlet so as to effectively prevent ice build up in the heat exchanger. At the same time, the flow control arrangement is of an extremely simple and noncomplex structure, is economical to manufacture, is simple in operation, and requires little if any maintenance.
In the improvement according to the present invention, the heat exchanger is of a boxlike configuration and includes a first pair of generally parallel opposite sides which respectively define the cold air inlet and discharge, and includes a second pair of generally parallel opposite sides which define the warm air inlet and discharge, the second sides being generally perpendicular with respect to the first sides. A plurality of heat exchanger dividers, preferably parallel plates, extend transversely across the heat exchanger and define a series of first channels for permitting solely flow of cold between the respective inlet and discharge, and a series of second channels which permit solely flow of warm air between the respective inlet and discharge, with the first and second channels being isolated from one another and alternately interposed to permit efficient heat transfer from the warm air through the heat exchanger plates to the cold air. A flow control device is disposed adjacent the cold air inlet in the vicinity of the corner between the cold air inlet and the warm air discharge to prevent ice build up at this corner of the heat exchanger. The flow control device includes a swingable platelike baffle which is positioned adjacent the corner when in a fully open position, and which is swingable away from the corner so as to partially close off a selected area of the cold air inlet directly adjacent the corner. This baffle, when in a partially closed position, deflects the inflowing cold air at the cold air inlet away from the aforementioned corner to prevent or significantly minimize any tendency for ice to build up at this corner. The baffle is controlled by a suitable drive motor, which in turn is controlled by an appropriate control or sensor so as to maximize system performance. The sensor preferably responds to and controls in accordance with outside air temperature. Even when the baffle is in a partially closed position so that the incoming cold air is diverted into only a portion of the cross sectional area of the cold air inlet, nevertheless the first channels as defined between adjacent heat exchanger plates are fully opened across the complete cross section thereof so that the incoming cold air, after passing downstream of the baffle plate, can expand outwardly so as to completely occupy the complete cross section of the first channels, which expansion occurs as the cold air flows longitudinally through the channels so that the volume of col air in the vicinity of the aforementioned corner is minimized, and build up of ice can be prevented or minimized. While the heat exchanger experiences a small decrease in air output and overall efficiency, nevertheless the heat exchanger is able to function in a continuous and reliable manner without experiencing drastic changes in efficiency and flow such as is experienced when bypassing of cold air is required for defrosting purposes.
Other objects and purposes of the present invention will be apparent to persons familiar with systems of this general type upon reading the following specification and inspecting the accompanying drawings.